There are sorbent materials, such as lime and limestone which are conventionally used in scrubbers to remove SO.sub.2 from combustion gases. One of the problems however with lime or limestone scrubbing is the resultant chemical scaling in the scrubber process vessels, transfer lines and flue system due to the deposition of solid deposits such as gypsum. Thus, there is an additional cost involving costly clean-up problems, particularly for systems in which the scrubbing system is utilized in a calcium-based, wet flue gas desulfurization process. The severity of the scaling problem is related to the concentrations of calcium and sulfate in the scrubbing solution, the former being present as a result of dissolution of the lime or limestone and the latter of which results from the absorption of SO.sub.2 and subsequent oxidation of dissolved sulfite. To reduce or eliminate the oxidation of sulfite to sulfate, thiosulfate has been utilized by reacting with free radicals to break the free radical chain reactions involved in the oxidation of sulfite to sulfate. These free radical chain reactions are also believed to be initiated by trace amounts of metal ion catalysts, such as iron or manganese, and by dissolved oxygen. The oxidation initiation of sulfite to sulfate, and to some extent the overall oxidation rates, are therefore functions of the liquid phase sulfite, dissolved oxygen, and catalyst concentrations. In addition, higher saturation temperatures which occur in flue gas desulfurization (FGD) systems in high moisture lignite fired boilers, also tend to increase oxidation.
In full scale flue gas desulfurization systems gypsum scaling is most likely to be a problem when the oxidation of sulfite to sulfate is above about 15% of the absorbed SO.sub.2, and usually within the range of 15 to 40% of the absorbed SO.sub.2. In this range calcium sulfate will precipitate both in the solid solution and as gypsum. Often there are not sufficient gypsum seed crystals to serve as precipitation sites in the slurry, so the gypsum also precipitates on vessels and pipe walls, forming scale deposits. However, when the oxidation is below 15%, particularly below about 10% of the absorbed SO.sub.2, the gypsum relative saturation (defined as the product of the calcium and sulfate ion activities divided by the solubility product constant for calcium sulfate) is significantly below 1.0, and there are generally no gypsum scaling problems within the scrubber system.
One problem, however, is that there is not a consistent level of thiosulfate which can be predetermined for use in a slurry which will be effective to lower the oxidation rate below 15%. In some instances, the oxidation rate may be reduced below 5% with thiosulfate levels of a few hundred ppm, however, in other instances the oxidation rate is reduced to only about 15% even with thiosulfate levels of several thousand ppm. There exist conditions, therefore, where thiosulfate alone is only marginally effective in inhibiting sulfite oxidation.
It is thus an object of the present invention to provide an improved method for wet, calcium-based FGD scrubbing using wet lime/limestone and thiosulfate which results in improved reagent utilization while still maintaining, or even improving, the benefits of thiosulfate by reducing formation of gypsum scale, providing better solids handling properties and efficient SO.sub.2 removal.
This and other objects of the invention will be apparent from the following description and from practice of the invention.